1. Field of the Invention
This invention, generally relates to flat panel displays and, more particularly, to plasmonic polarizer suitable for use in a flat panel display.
2. Description of the Related Art
The full range of colors produced by plasmon resonances resulting from metal nanostructures has been known since ancient times as a means of producing stained colored glass. For instance, the addition of gold nanoparticles to otherwise transparent glass produces a deep red color. The creation of a particular color is possible because the plasmon resonant frequency is generally dependent upon the size, shape, material composition of the metal nanostructure, as well as the dielectric properties of the surroundings environment. Thus, the optical absorption and scattering spectra (and therefore the color) of a metal nanostructure can be varied by altering any one or more of these characteristics. The parent applications listed above describe means of electronically controlling these color-producing characteristics.
The properties of metallic nanoparticles have drawn significant attention due to their application in photonics and electro-optics, as well as their potential application in biological/chemical sensors and renewable energy. Moreover, the fabrication of periodic metallic nanoparticle arrays for applications in photonics utilizing their localized surface plasmon resonance (LSPR) properties has been extensively studied in recent years. Among various processing techniques, depositing a film of metal on a nano-size patterned mask and using a lift-off process to remove the sacrificial layer is becoming a widely used technique, because it allows for fabricating nanoparticles with precisely controlled shape, size, and particle spacing. Moreover, advanced research has revealed that ordered array nanostructures have improved quantum characteristics utilized in LSPR properties, or photoluminescence and electroluminescence properties of semiconductor nanophosphors. Therefore, a method to achieve ordered nanoparticles and nanostructures is of significant importance.
A typical liquid crystal display (LCD) used in a laptop or cellular phone requires internal (backlight) illumination to render a color image. In polymer-networked liquid crystal (PNLC) or polymer dispersed liquid crystal (PDLC) devices, liquid crystals are dissolved or dispersed into a liquid polymer followed by solidification or curing of the polymer. During the change of the polymer from a liquid to solid, the liquid crystals become incompatible with the solid polymer and form droplets throughout the solid polymer. Typically, the liquid mix of polymer and liquid crystals is placed between two layers of glass or plastic that includes a thin layer of a transparent, conductive material followed by curing of the polymer. This structure is in effect a capacitor.
Electrodes from a power supply are attached to the transparent electrodes. With no applied voltage, the liquid crystals are randomly arranged in the droplets, resulting in scattering of light as it passes through the window assembly. This scattering results in a translucent “milky white” appearance. When a voltage is applied to the electrodes, the electric field formed between the two transparent electrodes on the glass causes the liquid crystals to align, allowing light to pass through the droplets with very little scattering and resulting in a transparent state. The degree of transparency can be controlled by the applied voltage.
FIG. 1 is a partial cross-sectional view of a liquid crystal cell assembly structure (prior art). A polarizer is a type of optical filter that selectively passes light of specific polarization in one direction and blocks waves of light in other polarizations. Polarizers are currently used in many optical systems, but among the most popular, with the largest world-wide market, is the use of polarizers in LCDs, including LCD TVs, monitors, note PCs, tablet PCs, and mobile/smart phones. For example, The bottom polarizer polarizes light from the backlight in one direction, say, the x-direction, in a horizontal plane parallel to the surfaces of the display. The top polarizer (above the color filter, in FIG. 1) is aligned perpendicular to the first polarizer, but also in the horizontal plane. It blocks light which passes through the first polarizer unless the polarization is rotated by the liquid crystal cell. Most of the linear polarizers used today in LCD applications are organic-based absorptive polarizers that are placed outside of two glass substrates that assemble a liquid crystal cell.
The absorptive film polarizer used in conventional mainstream LCD displays is made from an arrangement of iodine-doped polyvinyl alcohol (PVA) plastic that is stretched across the sheet during manufacturing to ensure that the PVA chains are aligned in one particular direction. Electrons from the iodine dopant are able to travel along the chains, ensuring that light polarized parallel to the chains is absorbed by the sheet, in contrast, light polarized perpendicular to the chains is transmitted generating a linear polarization. The liquid crystal acts as an optical switching mechanism to switch the backlight from parallel to perpendicular polarization by means of voltage actuation. However, some of the drawbacks to the conventional film polarizer are as follow: (1) a low optical efficiency with transmission values of 25˜30% for unpolarized light and 50˜60% for polarized light in the visible wavelength regime, with a typical extinction ratio value (Tper/Tpar) of ˜6000, (2) sensitivity to heat and humidity due to a film resin made from organic materials, and (3) limited display integration capability due to the need for a protective film such as tri-acetyl cellulose (TAC) film on both sides of polarizing layer. Specifically, thermal resistance is a significantly important issue to solve especially in projection-type liquid crystal displays where light of high energy density is introduced to both the liquid crystal and polarizer, with the latter having a function to absorb most of the unnecessary incident light.
FIG. 2 shows a schematic diagram of a glass polarizer with shape anisotropic metallic nanoparticles embedded in an optically transparent glass substrate (prior art). To overcome the problem of thermal resistance, a new type of polarizer made from glass substrates has been introduced to perform at long visible wavelengths to IR wavelengths (U.S. Pat. No. 7,019,903, U.S. Pat. No. 7,468,148, U.S. Pat. No. 8,063,999, and 2010/0019740). In these patents, the glass polarizer consists of fine metallic particles having shape anisotropy oriented and dispersed in a transparent glass substrate. Polarization is achieved from anisotropic plasmon resonances of absorption and transmission characteristics due to the embedded metallic particles.
FIG. 3 is a graph depicting the transmission characteristics of a glass polarizer made with nanoparticles having a shape anisotropy, see U.S. Pat. No. 8,063,999 (prior art). The absorption and transmission characteristics from plasmon resonances are due to correlations between the polarization plane of incident light and metallic nanoparticles having shape anisotropy. When the polarization plane is parallel to the long-axis direction of nanoparticles, the absorption peak is observed at longer wavelength (˜570 nanometers (nm)) characterized by the dip in transmission spectrum shown as a solid line. On the other hand, when the polarization plane is crossed (perpendicular) to the long-axis direction of nanoparticles, the absorption peak is observed at s shorter wavelength (˜390 nm) characterized by the dip in transmission spectrum as shown by the dotted line. The extinction ratio is represented as the ratio of cross transmission over the parallel transmission.
The glass polarizer tends to be more durable and stable in harsh environments, and polarizes light much better than conventional organic film polarizer, achieving extinction ratios as high as 100,000:1 for near infrared wavelength light. This type of glass polarizer is widely used as optical isolators in optical fiber communication. However, some of the drawbacks of glass polarizer are as follow: (1) a high extinction ratio is only achievable in long visible to IR wavelength due to proprietary glass manufacturing processes, and (2) a low integration capability into devices such as in LCD pixel cell due to large thickness of glass substrate.
FIG. 4 is a graph depicting transmittance as a function of wavelength (prior art). U.S. Pat. No. 8,063,999 describes a glass polarizer that operates in broadband visible and near infrared wavelengths. The cross transmission spectrum and extinction ratio of the glass polarizer are shown. These characteristics are plotted on the graph for wavelengths in the range of 400 nm to 2000 nm.
Even for the broadband glass polarizer shown in FIG. 4, there is still a sharp cut off in transmission at wavelengths below 500 nm. Moreover, the extinction ratio drops to 30 dB, which translates to only 1000:1 transmission contrast ratio at λ=500 nm. Therefore, a new type of polarizer is needed that can overcome these challenges and perform well at shorter visible wavelengths of 500 nm or below, in order to practically incorporated into display applications.
FIG. 5 shows the schematic structure of an LCD device with the polarizer placed outside of the LC substrates (prior art). In US 2010/0019740, Kadowaki et al. disclose a liquid crystal panel display that uses phosphor layers as color generating pixels that features better visual performance with wider viewing angle, as compared to conventional LCDs that use color filters and a cold-cathode fluorescent lamp (CCFL) backlight source. In FIG. 5, the two linear film polarizers (14, 15) are placed directly outside of liquid crystal cell 13, laminated onto glass substrates 11 and 12, which is the same approach used in conventional LCDs. However, in contrast to the conventional LCD, the realization of PLD device structure requires another glass substrate (not explicitly shown) that is incorporated as part of the phosphor/color filter layers 17. After manufacturing of phosphor layers onto this substrate, the phosphor substrate and liquid crystal cell are assembled together to make the display device. With this configuration, an unnecessary gap spacing is created between LC cell and phosphor layers, which causes optical cross-talk within the system, and therefore creates low contrast and poor visual performances in the display. Also, the manufacturing costs would be significantly high due to the complex 3-glass substrate structure.
It would be advantageous if a polarizer could be fabricated with a thickness less than 10 microns.
It would be advantageous if a polarizer could be fabricated using inorganic materials, for enhanced reliability.
It would be advantageous if a polarizer could be fabricated with a large extinction ratio at wavelengths of 500 nm and below.